Liquid heater including wire mesh heating segment

ABSTRACT

There is provided a liquid heater including: a conduit; a circuit for carrying a DC current; and a wire mesh segment disposed in the conduit and configured to receive the current, wherein the wire mesh segment has a conically shaped surface. There is provided a liquid heater kit including: a DC power supply; and a wire mesh segment configured to be disposed in a conduit and to receive a current from the DC power supply, wherein the wire mesh segment has a conically shaped surface.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a 371 of International Patent Application No. PCT/US2014/023954 filed Mar. 12, 2014, which claims the benefit of U.S. Provisional Application No. 61/801,028, filed Mar. 15, 2013, all of which are incorporated in their entirety by reference for all purposes as if fully set forth herein.

BACKGROUND OF THE INVENTION Field of the Invention

Exemplary embodiments of the present invention relate to a liquid heater that includes a wire mesh segment. The wire mesh segment can include a Nichrome wire. Nichrome wire is commonly used in appliances such as hair dryers and toasters as well as used in embedded ceramic heaters. The wire has a high tensile strength and can easily operate at temperatures as high as 1250 degrees Celsius. Nichrome has the following physical properties (Standard ambient temperature and pressure used unless otherwise noted):

Material property Value Units Tensile Strength 2.8 × 10⁸ Pa Modulus of elasticity  2.2 × 10¹¹ Pa Specific gravity 8.4    None Density 8400 kg/m³ Melting point 1400 ° C. Electrical resistivity at room  1.08 × 10^(−6[1]) Ω · m temperature Specific heat  450 J/kg° C. Thermal conductivity    11.3 W/m/° C. Thermal expansion   14 × 10⁻⁶ m/m/° C.

SUMMARY OF THE INVENTION

According to various embodiments, there is provided a liquid heater including: a conduit; a circuit for carrying a DC current; and a wire mesh segment disposed in the conduit and configured to receive the current, wherein the wire mesh segment has a conically shaped surface.

According to various embodiments, there is provided a liquid heater kit including: a DC power supply; and a wire mesh segment configured to be disposed in a conduit and to receive a current from the DC power supply, wherein the wire mesh segment has a conically shaped surface.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. Other features and aspects will be apparent from the following detailed description, the drawings, and the claims.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention, and together with the description serve to explain the principles of the invention.

The same reference number represents the same element on all drawings. It should be noted that the drawings are not necessarily to scale. The foregoing and other objects, aspects, and advantages are better understood from the following detailed description of an embodiment with reference to the drawings, in which:

FIG. 1 is a graph illustrating the radiative area of a mesh element as a function of the center to center spacing of the mesh strands.

FIG. 2 is a graph illustrating the electrical resistance of a mesh element as a function of the radius of the strand and the mesh spacing.

FIG. 3 is a graph illustrating the ramp up time of a two sided 125 mm×250 mm mesh element oven as a function of the radius of the strand and the mesh spacing and power drain of 20 KW.

FIG. 4 is a composite graph of FIG. 1 and FIG. 2, indicating the regions applicable for high speed oven cooking with a De Luca Element Ratio close to 0.11 ohms/m2.

FIG. 5 illustrates a liquid heater including a wire mesh heating element according to various embodiments.

FIG. 6 illustrates a top down view of a liquid heater including a wire mesh heating element according to various embodiments.

FIG. 7 illustrates a bottom up view of a liquid heater including a wire mesh heating element according to various embodiments.

DESCRIPTION

The invention is described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure is thorough, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the size and relative sizes of layers and regions may be exaggerated for clarity. Like reference numerals in the drawings denote like elements.

It will be understood that when an element is referred to as being “connected to” another element, it can be directly connected to the other element, or intervening elements may be present.

When considering the use of Nichrome within an oven it is important to consider not only the resistive characteristics but also the black body emission of the element when hot.

With Regard to the General Characterization of Resistive Elements, the resistance is proportional to the length and resistivity, and inversely proportional to the area of the conductor.

R=L/A·ρ=L/A·ρ ₀(α(T−T ₀)+1)   Eq.1

where ρ is the resistivity:

ρ=1/σ.

L is the length of the conductor, A is its cross-sectional area, T is its temperature, T0 is a reference temperature (usually room temperature), ρ0 is the resistivity at T0, and α is the change in resistivity per unit of temperature as a percentage of ρ0. In the above expression, it is assumed that L and A remain unchanged within the temperature range. Also note that ρ0 and α are constants that depend on the conductor being considered. For Nichrome, ρ0 is the resistivity at 20 degrees C. or 1.10×10−6 and α=0.0004. From above, the increase in radius of a resistive element by a factor of two will decrease the resistance by a factor of four; the converse is also true.

Regarding the power dissipated from a resistive element, where, I is the current and R is the resistance in ohms, v is the voltage across the element, from Ohm's law it can be seen that, since v=iR,

P=i²R

In the case of an element with a constant voltage electrical source, such as a battery, the current passing through the element is a function of its resistance. Replacing R from above, and using ohms law,

P=v ² /R=v ² A/ρ ₀ L   Eq. 2

In the case of a resistive element such as a nichrome wire the heat generated within the element quickly dissipates as radiation cooling the entire element.

Now, Considering the Blackbody Characterization of the Element: Assuming the element behaves as a blackbody, the Stefan-Boltzmann equation characterizes the power dissipated as radiation:

W=σ·A·T ⁴   Eq. 3

Further, the wavelength λ, for which the emission intensity is highest, is given by Wien's Law as:

λ_(max) =b/T   Eq. 4

0 Where,

σ is the Stefan-Boltzmann constant of 5.670×10⁻⁸ W·m⁻²·K⁻⁴ and,

b is the Wien's displacement constant of 2.897×10−3 m·K.

In an application such as a cooking oven, requiring a preferred operating wavelength of 2 microns (2×10E−6) for maximum efficiency, the temperature of the element based on Wein's Law should approach 1400 degrees K. or 1127 degrees C. From the Stefan-Boltzmann equation, a small oven with two heating sides would have an operating surface area of approximately 4×0.25 m×0.25 m or 0.25 m2. Thus, W should approach 20,000 Watts for the oven.

In the case of creating a safe high power toaster or oven it is necessary for the system to operate at a low voltage of no more than 24 volts. Thus, using Eq. 2 with 20,000 W, the element will have a resistance of approximately 0.041 ohms, if 100% efficient at the operating temperature. Based on Eq. 1, a decrease in operating temperature to room temperature (from 1400 to 293 k) represents an approximate decrease in the resistivity of the element by about 1.44 times, and therefore an element whose resistance at room temperature is 0.0284 ohms is required.

Now, Considering the Relationship of the Resistance of the Element and the Characterization of the Element as a Blackbody:

The ratio of the resistance of the heater to the black body raditive area of the same heater becomes the critical design constraint for the oven; herein termed the De Luca Element Ratio. The ideal oven for foods operating over a 0.25 square meter area at 2 micron wavelength has a De Luca Element Ratio (at room temperature), of 0.1137 ohms/m2 (0.0284 ohms/0.25 m2). The De Luca Element Ratio is dependent solely on the resistance of the material and the radiative surface area but is independent of the voltage the system is operated. In addition, for wire, the length of the wire will not change the ratio.

Table 1 lists the resistance per meter of several common nichrome wire sizes as well as the De Luca Element Ratio for these elements. It is important to note that all these wires have a De Luca Element Ratio far greater than the 0.1137 required for an oven operated at 1400K, 24V, and over 0.25 m2. Clearly the use of a single wire with a voltage placed from end-to-end in order to achieve the power requirement is not feasible.

In contrast, a household pop-toaster, operated at 120V and 1500 W, over a smaller 0.338 m2 area at 500K would require a De Luca Element Ratio of 35.5. Thus a 1 meter nichrome wire of 0.001 m radius with a 120V placed across it would work appropriately.

TABLE 1 Surface De Luca Resistance Area of 1 Element Time To Cross Per Meter meter Weight Ratio (at Reach Wire Sectional Length length Per Meter room 1400 K At Radius (m) Area (m2) (ohms) (m2) (g) temp) 20 kw (sec) 0.01  3.14E−04 0.0034 0.0628 2637 0.1 65.4 0.0015  7.06E−06 0.15 0.00942 59.3 16.2 1.47 0.001  3.14E−06 0.30 .00628 26.3 47.7 0.654 .0005  7.85E−07 1.38 .00314 6.6 438 0.163 0.000191 1.139E−07 11.60 0.00120 0.957 9670 0.024 0.000127 5.064E−08 24.61 0.00079 0.425 30856 0.010 0.000022 1.551E−09 771.21 0.000138 0.013 5580486 0.0003

Clearly a lower resistance or a higher surface area is required to achieve a De Luca Element Ratio of close to 0.1137.

One way to achieve the De Luca Ratio of 0.1137 would be to use a large element of 2 cm radius. The problem with this relates to the inherent heat capacity of the element. Note from Table 1 that to raise the temperature to 1400K from room temperature would require 65.4 seconds and thus about 0.36 KWH of energy.

This Calculation is Derived from the Equation Relating Heat Energy to Specific Heat Capacity, where the Unit Quantity is in Terms of Mass is:

ΔQ=mcΔT

where ΔQ is the heat energy put into or taken out of the element (where P×time=ΔQ), m is the mass of the element, c is the specific heat capacity, and ΔT is the temperature differential where the initial temperature is subtracted from the final temperature.

Thus, the time required to heat the element would be extraordinarily long and not achieve the goal of quick cooking times.

Another way for lowering the resistance is to place multiple resistors in parallel. Kirkoffs law's predict the cumulative result of resistors placed in parallel.

$\begin{matrix} {\frac{1}{R_{total}} = {\frac{1}{R_{1}} + \frac{1}{R_{2}} + \ldots + \frac{1}{R_{n}}}} & {{Eq}.\mspace{14mu} 5} \end{matrix}$

The following Table 2 lists the number of conductors for each of the elements in Table 1, as derived using equation 5, that would need to be placed in parallel in order to achieve a De Luca Element Ratio of 0.1137. Clearly placing and distributing these elements evenly across the surface would be extremely difficult and impossible for manufacture. Also note that the required time to heat the combined mass of the elements to 1400K from room temperature at 20 KW for elements with a radius of greater than 0.0002 meters is too large with respect to an overall cooking time of several seconds.

TABLE 2 Number of De Luca Parallel Element Elements Ratio Required to Total Time To Reach Wire for single Achieve De Weight/ 1400 K At 20 kw Radius element (@ Luca Ratio of Meter (sec) From (m) Room Temp) 0.1137 (g) Room Temp 0.01 0.1 1 2637 65.4 0.0015 16.2 12 711 17.6 0.001 47.7 22 579 14.4 .0005 438 63 415 10.3 0.000191 9670 267 255 6.3 0.000127 30856 493 209 5.2 0.000022 5580486 6838 88 2.18

In summary, the following invention allows for the creation of a high power oven by using a resistive mesh element. The heater element designed so as to allow for the desired wavelength output by modifying both the thickness of the mesh as well as the surface area from which heat radiates. The heater consisting of a single unit mesh that is easily assembled into the oven and having a low mass so as to allow for a very quick heat-up (on the order of less than a few seconds).

Specifically, the wire mesh cloth design calibrated to have the correct De Luca Element Ratio for a fast response (less than 2 sec) oven application operating at 1400 degrees K.

According to exemplary embodiments, a mesh design for operating a quick response time oven consisting of a nichrome wire mesh with strand diameter of 0.3 mm, and spacing between strands of 0.3 mm, and operating voltage of 24V.

In considering the best mesh design, it is important to evaluate the blackbody radiative area as well as the resistance of the element as a function of the following:

1) The number of strands per unit area of the mesh

2) The radius of the mesh strands

3) The mesh strand material

4) The potential for radiation occlusion between strands.

FIG. 1 describes the blackbody area as a function of the number of strands and the strand spacing of the mesh. Interestingly, the surface area is independent of the radius of the wire strand if the spacing is made a function of the radius.

Using equation 5 from above, the resistance of the mesh can be calculated for a specific wire strand radius. FIG. 2 illustrates the electrical resistance of a nichrome mesh element as a function of the radius of the strand and the mesh spacing. Limitation in Equation 5 become apparent as the number of strands becomes very high and the resistance becomes very low; thus atomic effects associated with random movement of electrons in the metal at room temperature form a minimum resistive threshold.

Using nichrome as the strand material in the mesh and operating the system at 20 KW, the ramp up time to achieve an operating temperature of 1400 degrees K. is a function of the strand radius and the mesh spacing (note that a nominal mesh size of two times 125 mm×250 mm is used). FIG. 3 illustrates the region below which a ramp up of less than 2 seconds is achievable (note that wire radius above 0.5 mm are not shown due to the long required ramp up times).

FIG. 4 is a composite graph of FIGS. 1 and 2, indicating the regions applicable for high speed oven cooking with a De Luca Element Ratio close to 0.11 ohms/m2.

A liquid heater including a wire mesh segment wherein the liquid to be heated flows in the voids in the mesh, i.e., between the wire segments that form a wire mesh, is described. The liquid can be continuously heated as it flows. The liquid can be flash or instantly heated. As such, the present liquid heater can be disposed proximate or adjacent to a point of use or consumption.

Various embodiments that can raise the temperature of a liquid over a large range can be provided. Various embodiments that can raise the temperature of a large quantity of liquid can be provided. In some embodiments, multiple liquid heaters can be disposed in series, i.e., one after another in a conduit like a pipe. In some embodiments, multiple liquid heaters can be disposed in parallel conduits.

Each wire mesh segment or heating element can be individually controlled for intensity and/or duration. This embodiment can provide the advantage of heating or cooking with a high flow rate. In addition, the heating profile for each wire mesh segment can be optimally customized. The customization can be achieved without reconfiguring the hardware of the liquid heater.

Each length of a wire mesh segment and intervening gaps between lengths of the wire mesh segments can provide the equivalent effect of an on-and-off pulsed liquid heater. This can permit for a continuous process flow, for example, when showering, filling a tub, or otherwise demanding a liquid at a high rate of flow.

In some embodiments, a flow rate of the liquid runs at a constant speed. As the liquid to be heated flows forward, the wire mesh segments can heat the liquid. A wire mesh segment or heating element either may be already on or may turn on when a flow is detected. In the absence of a liquid flow, the wire mesh heating element can be turned off. As it flows, the liquid flow passes through the voids in the wire mesh segment and heats. In some embodiments, the spacing between the strands of wire forming the wire space can be covered or blocked. As such, when the spacing between the strands of wire forming the wire space is blocked or a wire mesh segment is not porous, the water flows along the surface of the wire mesh segment.

In some embodiments, after the liquid flows past a wire mesh element, the liquid can be cooled. A duration of a cool-off period can be achieved with a gap between adjacent wire mesh segments. In some embodiments, the wire mesh element includes a Ni-Chrome heating element.

In some embodiments, the wire mesh heater can be disposed in a conduit. In some embodiments, the wire mesh heater can be integrated or be formed as a unitary construction. The conduit can include industry standard male or female fittings. As such, the heater can be disposed in plumbing, for example, household plumbing.

The conduit can include a leak proof nipple or fitting. Electrical leads connected to the wire mesh segment can exit from the conduit from the leak proof nipple. In some embodiments, shielding to reflect infrared radiation can be provided on an inner surface of a conduit in which a wire mesh heater is disposed.

The conduit can include one or more temperature sensors. In some embodiments, the temperature sensor can be disposed downstream of the water mesh heater. The temperature sensors can be disposed upstream of the water mesh heater.

A controller that reads a temperature signal from a temperature sensor can be provided. The controller can limit the temperature of the heated liquid, for example, by turning off and on the DC power supply.

The conduit can include a liquid flow sensor. The flow sensor can enable the DC power supply when a flow is present. In some embodiments, the flow sensor signal the controller whether a liquid flow is present or not.

The conduit can include a label indicating the direction of the liquid flow.

A low-voltage Direct Current (DC) power supply to energize the water-heating element can be provided. Exemplary low voltages include 6 Volts (V), 12 V, 18 V, 24 V, and the like. The DC power supply can be a high-amperage power supply.

A liquid heater kit can include a conduit including a wire mesh segment and a DC power supply to be connected to the wire mesh segment. The kit can include a heat insulator disposed on an outer surface of the conduit. The kit can include a controller. The kit can include a flow sensor. The kit can include a temperature sensor.

FIG. 5 illustrates a liquid heater including a wire mesh heating element according to various embodiments. A liquid heater 100 can include a conduit 150. The conduit 150 can include threads 152. Disposed within the conduit 150 can be a wire mesh segment 114.

In some embodiments, the wire mesh segment 114 can have a conical shape. The generally conical surface of the wire mesh segment 114 can further include ridges or crests and dips or valleys to increase a surface area of the conical surface. For example, the ridges and dips can be formed in a sinusoidal shape. The ridges and dips can be disposed along a length of the wire mesh segment 114. In some embodiments, the ridges and dips can be disposed along a partial length of the wire mesh segment 114.

A broad radius ring 110 of wire mesh segment 114 can be disposed upstream of a narrow radius ring 112 along a flow 124. In some embodiments, the orientation of the flow can be reversed, i.e., the broad ring 110 can be disposed downstream of the narrow ring 112. Electrical lead 120 can be connected to the broad ring 110. Electrical lead 122 can be connected to the narrow ring 112. In some embodiments, electrical lead 120 can be connected to a positive electrode of a DC power supply 128. In some embodiments, electrical lead 122 can be connected to a negative electrode of a DC power supply 128. In some embodiments, the negative electrode can be connected to the broad ring 110, and the positive electrode can be connected to the narrow ring 112.

Wire mesh segment 114 can be secured to conduit 150 near or through the broad ring 110 using, for example, a screw 116. Wire mesh segment 114 can be secured to conduit 500 near or through the narrow ring 112 using, for example, a screw 116. Other means known in the art can be used to secure the wire mesh segment 114 to conduit 150 using, for example, an adhesive, a rivet, using soldering, using brazing, using welding, and the like.

A controller 129 can receive a signal from a temperature sensor 124. Temperature sensor 124 can be disposed downstream of wire mesh segment 114. Temperature sensor 124 can measure the temperature of the heated liquid. The controller 129 can include an input that can set the max temperature of the heated liquid. The input can, for example, include a dial, a knob, or any other input means known in the art. The controller 129 can sense a temperature signal from the temperature sensor124 and control the wire mesh segment 114 based on the max temperature input. In some embodiments, the controller 129 can enable an electrical connection between the wire mesh segment 114 and the power supply 128 when the temperature is below a max temperature. In some embodiments, the controller 129 can disable an electrical connection between the wire mesh segment 114 and the power supply 128 when the temperature is at or above a max temperature.

A controller 129 can receive a signal from a flow sensor 126. Flow sensor 126 can be disposed upstream of wire mesh segment 114. Temperature sensor 124 can measure a flow of the liquid, for example, the unheated liquid. The controller 129 can sense the signal from the flow sensor 126 and control the wire mesh segment 114, by enabling an electrical connection to the power 128 when a flow is sensed. In some embodiments, the controller 129 can disable an electrical connection between the power supply 128 and wire mesh segment 114.

The present disclosure allows for the creation of a high power liquid heater by using a resistive wire mesh element. The heater element can allow for a desired wavelength output by modifying both the thickness of the mesh as well as the surface area from which heat radiates. The heater includes a single unit mesh that is assembled into a liquid/water heater and having a low mass so as to allow for a very quick heat-up (on the order of less than a few seconds).

Wire mesh segment can include horizontal and vertical wires crisscrossing one another. The nodal intersections of the wires can form an electrical short. The wire mesh need not be electrically insulated. In some embodiments, the wire mesh includes an electrical insulator disposed thereupon. The wire mesh wire can include Nichrome.

In some embodiments, the wire mesh can include a hydrophilic coating in order to facilitate movement of heated water away from the wire mesh. A mesh design for operating a quick response time liquid heater can include of a nichrome wire mesh with strand diameter of 0.3 mm, and spacing between strands of 0.3 mm, and operating voltage of 24V. In some embodiments, the wire mesh can have a strand diameter of, for example, less than 1.5 mm, 1 mm, less than 0.7 mm, less than 0.5, less than 0.3 mm, less than 0.1 mm, or the like. In some embodiments, the spacing between strands can have a length of, for example, less than 1.5 mm, 1 mm, less than 0.7 mm, less than 0.5, less than 0.3 mm, less than 0.1 mm, or the like.

The wire mesh can include a wire mesh cloth that is, for example, calibrated for a fast response heating application operating. For example, the wire mesh can operate at 1400 degrees K or greater. In some embodiments, the wire mesh can attain a high temperature in, for example, less than 10 seconds, in less than 5 seconds, in less 2 seconds or the like.

The DC power supply can operate at, for example, 24 V or less, 12 V or less, 6 V or less, or the like.

A length L of the wire mesh can be, for example, less than 200 mm, less than 150 mm, less than 100 mm, less than 50 mm, or the like.

FIG. 6 illustrates a top down view of a liquid heater including a wire mesh heating element according to various embodiments.

FIG. 7 illustrates a bottom up view of a liquid heater including a wire mesh heating element according to various embodiments.

The examples presented herein are intended to illustrate potential and specific implementations. It can be appreciated that the examples are intended primarily for purposes of illustration for those skilled in the art. The diagrams depicted herein are provided by way of example. There can be variations to these diagrams or the operations described herein without departing from the spirit of the invention. For instance, in certain cases, method steps or operations can be performed in differing order, or operations can be added, deleted or modified. 

What is claimed is:
 1. A liquid heater comprising: a conduit; a circuit for carrying a DC current; and a wire mesh segment disposed in the conduit and configured to receive the current, wherein the wire mesh segment has a conically shaped surface.
 2. The liquid heater of claim 1, wherein the DC current is supplied by a voltage source having a potential less than 24 Volts.
 3. The liquid heater of claim 1, wherein the wire mesh segment comprises multiple wire mesh segments disposed in the conduit.
 4. The liquid heater of claim 3, further comprising a relay for cycling the current connection to each of the multiple wire mesh segments, and a control circuit for controlling each of the relays.
 5. The radiant oven of claim 3, further comprising: a control circuit for controlling current to each of the multiple wire mesh segments by cycling on and off at a duty ratio in response to a user input, or automatically in response to a measured parameter indicting a condition of a liquid.
 6. The liquid heater of claim 1, further comprising a voltage control circuit configured for varying the voltage of each of the multiple wire mesh segments.
 7. The liquid heater of claim 1, wherein the wire mesh segment comprises multiple wire mesh segments disposed in the conduit, and electrically connected to a DC power supply in a parallel circuit.
 8. The liquid heater of claim 1, wherein the conical surface comprises ridges and dips.
 9. The liquid heater of claim 8, wherein the ridges and dips form a sinusoidal shape.
 10. The liquid heater of claim 1, further comprising: a temperature sensor to generate a liquid temperature signal; and a controller to sense the liquid temperature signal, wherein the circuit for carrying the DC current is enabled when the liquid temperature signal indicates a temperature less than a desired temperature.
 11. The liquid heater of claim 1, further comprising: a flow sensor to generate a liquid flowing signal; and a controller to sense the liquid flowing signal, wherein the circuit for carrying the DC current is enabled when the liquid temperature signal indicates liquid flow.
 12. The liquid heater of claim 1, wherein the wire mesh segment comprises a broad ring configured to be connected to a positive electrode of a DC power supply.
 13. The liquid heater of claim 1, wherein the wire mesh segment comprises a narrow ring configured to be connected to a negative electrode of a DC power supply.
 14. The liquid heater of claim 1, wherein the wire mesh segment comprises a wire mesh cloth comprising wire strands having a diameter less than 0.5 mm and a spacing between the wire strands of less than 0.5 mm.
 15. The liquid heater of claim 14, wherein the wire strands crisscross and form an electrical short at an intersection.
 16. The liquid heater of claim 1, wherein the wire mesh segment comprises a hydrophilic coating.
 17. A liquid heater kit comprising: a DC power supply; and a wire mesh segment configured to be disposed in a conduit and to receive a current from the DC power supply, wherein the wire mesh segment has a conically shaped surface.
 18. The liquid heater kit of claim 17, further comprising a conduit, wherein the wire mesh element is disposed in the conduit.
 19. The liquid heater kit of claim 18, wherein the controller and the wire mesh are of a unitary construction.
 20. The liquid heater kit of claim 17, further comprising a controller and a temperature sensor. 